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Old 5th February 2007, 05:21 PM   #1
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Default how to measure hum

I don't know what happened to my original post!!

Anyway, no matter how I search for this I can't seem to get a straight answer.

How do I measure hum? Is it as simple as putting a DMM set to mV AC across the loudspeaker terminals with nothing connected to input?


What measurement should I get (I know zero would be great but in the real world whats OK)?

I'm awaiting my morgan jones books!

Fran
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Old 6th February 2007, 01:59 AM   #2
gofar99 is offline gofar99  United States
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Default Amp Hum

Hi, There are a number of ways to measure hum. I prefer using an oscilloscope. (hope you have access to one) You can use a sensitive meter, but there are two problems. First you only know that you are measuring a voltage, it could be noise, hum, part DC, or in the case of a recent amp of mine high frequency oscillation. The scope tells it all. Second depending on what is actually there the meter might not be accurate (not linear) at the frequency of the measured voltage(s). As for how much is too much, this is a personal factor. Zero is best but rather unusual. Solid state amps are often quieter than tubes. In either case I prefer to have less than 5millivolts of hum going to the speaker. That works out to under a milliwatt and unless you have rather sensitive speakers will not be audible except when you put your ear right up to the speaker.

My present tube amp has about 10-15 mv and when driving 87 db/w speakers is barely audible at the speaker. It is completely gone at anything over 6 inches.

Hope this helps
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Old 6th February 2007, 09:43 AM   #3
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Thanks,

I put my DMM across the speaker terminals last night and got 3.5mV one side and 5.7mV the other side. From what you're saying this would be low enough (and I don't hear the hum from my loudspeakers) but I can hear it from the headphone jack - which is basically connected into the valve output before the output transformers.

I'm awaiting some better tubes to see if this helps reduce it, and failing that I think my next step would be to convert the AC heater supply to DC.

Fran
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Old 6th February 2007, 03:11 PM   #4
SY is offline SY  United States
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If I make the unjustified assumption that you've got 8 ohm speakers, 1W is 2.83V. Your hum is 55-60dB below that. That's OK, but 1mV is easily attainable without DC heaters. A scope will help a LOT to tell you where to do the last bit of tweaking.
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Old 6th February 2007, 04:06 PM   #5
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SY,

you know this is to do with that chinese 6P1 amp that I put up a thread about a little while ago. The new mil-spec tubes haven't arrived yet, but I just wanted to put some figure on the hum I have now, and then again after putting in the new tubes....

An oscilloscope would be handy but I don't know of anyone with one, never mind whether I would be able to figure out exactly what to do with it!


stupid maybe, but sometimes having the figures really helps.

Fran
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Old 6th February 2007, 04:33 PM   #6
SY is offline SY  United States
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Compared to what one spends on tubes and transformers, a scope is pretty cheap and as necessary to amp design/building/tweaking as socket wrenches are for engine work. Without it, you're working blind.
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Old 6th February 2007, 05:33 PM   #7
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Can you give me some pointers as to what to look for? The picture I have in my head of an oscilloscope is of this kind of thing:

Click the image to open in full size.


Wouldn't these be expensive? I would've said 300+ ?

Maybe theres some simpler handheld modern ones or even gizmos that connect to a PC?

Fran
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Old 6th February 2007, 06:23 PM   #8
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Along these lines, I've been meaning to pick up a scope for ages. What are some reasonable recommendations if your budget is $250 or so? Also, are there any good tutorials or other information (books, whatever) that tell you how to effectively use one on tube gear? My only experience with scopes was years ago designing digital circuits.
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Old 6th February 2007, 06:31 PM   #9
gofar99 is offline gofar99  United States
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All I read above rings true with my experience. Two things stand out first a modest scope can be had on ebay for under $100US. It's a good investment if you intend to continue with audio changes. Second if you are getting the headphone signal off the tube plates, I see a number of problems. First how are you blocking the high voltage DC. This could be dangerous. Second the level of the hum will be higher as the overall voltage of the signal is higher (lower current though). This is why there is a transformer. I would take the signal off the output side of the transformer and use some sort of dividing network to bring it down to reasonable levels. I have seen simple networks using a series resistor of about 50-100 ohms and a second one parallel with the phones of about 8-20 ohms. I use a 50 ohm and 8 on my K-12 and it works fine.

Good listening
gofar99
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Old 6th February 2007, 06:32 PM   #10
SY is offline SY  United States
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We've had a PILE of threads on choosing a scope. Figure to spend $100-300 for something that will give you years of good service.
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